The present invention relates to apparatus and methods for generating electricity in an aircraft by using the low pressure spool of an engine and, more particularly, to apparatus and methods for using the auxiliary power unit during approach and taxi, in conjunction with the low pressure spool, to provide electricity to the aircraft.
A gas turbine engine generally includes one or more compressors followed in turn by a combustor and high and low pressure turbines. These engine components are arranged in serial flow communication and disposed about a longitudinal axis centerline of the engine within an annular outer casing. The compressors are driven by the respective turbines and compressor air during operation. The compressor air is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow through the high and low pressure turbines, which extract the energy generated by the hot combustion gases for driving the compressors, and for producing auxiliary output power.
The engine power is transferred either as shaft power, pneumatic power (bleed air) or thrust for powering an aircraft in flight. Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand over four times the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly.
Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. Extracting excessive power from the HP spool reduces engine surge margins and therefore requires increased engine speed at idle conditions. The increased engine speed results in excessive residual thrust. Surge bleed can be used to improve engine surge margin but this results in wasted energy and fuel.
Extraction of electrical power from the low pressure (LP) engine spool, rather than just the HP spool, is effective in maintaining surge margins without surge bleed. However, LP spool electrical power extraction is difficult due to the wide speed range (over 1:4 from ground idle to 100%). This wide speed range may make large electrical generators, such as those needed for aircraft relying on more electric architecture (MEA), impractical.
As can be seen, there is a need for a method or system to extract electrical power from the engine that will allow electrical power to be generated during low engine operating power levels without reducing engine operability, but will satisfy the increasing electrical demands of the aircraft.